The present invention relates to radio communication systems. More particularly, and not by way of limitation, the present invention is directed to a method and apparatus for determining downlink signaling power from a base station/Node B to a mobile station operating in a cellular radio communication network.
Wideband CDMA (WCDMA) is emerging as the leading global third generation (3G) standard. Specifications are evolving with the introduction of enhancements in the WCDMA uplink that are now part of the Third Generation Partnership Project (3GPP) Release 6. The main requirements driving this evolution are reduced delays, improved uplink high-data-rate coverage, and higher capacity. To meet these requirements, the following enhancements have been introduced: a short 2 ms Transmission Time Interval (TTI) for data transmissions, fast scheduling, and fast Hybrid Automatic Transmission Request (HARQ). To support these enhancements, a new uplink transport channel has been introduced, the Enhanced Dedicated Channel (E-DCH), in which a set of separate channelization codes is utilized for the data and the associated control signaling. The number of channelization codes carrying the E-DCH and their spreading factors depend on the data rate being utilized. The Enhanced Dedicated Physical Control Channel (E-DPCCH), carrying information for HARQ and transport format, uses a new code. These channels are code-multiplexed with the Dedicated Physical Data Channels (DPDCH) and Dedicated Physical Control Channels (DPCCH) of previous releases that use a 10 ms TTI for circuit switched services such as speech.
HARQ is one of the key enablers for meeting the WCDMA objectives with fast retransmission and soft combining. To support uplink HARQ operations, Acknowledgment Channels (ACKCHs), also known as E-DCH HARQ Indicator Channels (E-HICHs) in WCDMA, are needed in the downlink for the base station to signal Ack (Acknowledgment) or Nack (Not Acknowledgment) messages.
In addition to HARQ, transmission rate control is used to adjust cell-wide uplink interference (also known as uplink noise rise) so that the target cell-wide quality of service, in terms of delays, throughput, and/or call blockage can be met. To achieve this, two additional downlink signaling channels are introduced in WCDMA, namely the E-DCH Absolute Grant Channel (E-AGCH) and the E-DCH Relative Grant Channel (E-RGCH). E-AGCH provides fast signaling to adjust the maximum allowable transmit data rate for scheduled users, whereas E-RGCH is a 1-bit (or three-level) message sent within a TTI to fine-tune the transmit data rate of an active user.
To maximize the benefits of HARQ and rate control, these downlink-signaling channels need to be received with high reliability. To achieve this, the E-HICH, E-AGCH, and E-RGCH must be sent with sufficient power. However, using excessive power to send E-HICH, E-AGCH, and E-RGCH results in lower available power for data and voice communications, which leads to lower data throughput or voice capacity in the downlink. Thus, it is important to have a good trade-off between downlink signaling reliability and power consumption.
In previously disclosed solutions for this problem, an E-DPDCH user is typically assigned an associated dedicated physical channel (DPCH) in both the uplink and the downlink. This DPCH is mainly used to keep the power control loop working. With power control (both inner and outer loops), the transmit power of the DPCH is appropriately determined so that the target performance of the DPCH can be met. Thus, a simple solution for determining the transmit power of the E-HICH, E-AGCH, and E-RGCH is to apply an offset to the transmit power of the DPCH. For example, if the power control mechanism has set the transmit power of DPCH as PDPCH, and the nominal desired SINRs of the DPCH and the E-HICH are x and y, respectively, the transmit power of E-HICH (PEHICH) can be determined by:PEHICH=(y/x)PDPCH.
This scheme works well when the associated DPCH is not in the soft handoff (SHO) mode. During soft handoff, the power of the DPCH is decreased by a SHO gain. For signaling channels, however, SHO gain is not available because different active cells may send different downlink signaling messages. As a result, the transmit power of the E-HICH can be determined by:PEHICH=(zy/x)PDPCH,where z accounts for the SHO gain of the DPCH. Since the Radio Network Controller (RNC) knows whether an associated DPCH is in soft handoff mode or not, the RNC can signal the power adjustment factor, y/x or zy/x to the Node Bs.
Another known way to determine the transmit power of the E-HICH is based on the Channel Quality Indicator (CQI) report, which indicates the received signal-to-interference-plus-noise ratio (SINR) of the High-Speed Downlink Shared Channel (HS-DSCH) for a nominal power allocation, (Ec/Ior)HSDSCH, where the factor Ec/Ior is defined as the ratio of the transmit power utilized for a particular channel to the total transmit power of the base station. Expressed in another way:(γHSDSCH)dB=CQI+γ0,where (γHSDSCH)dB is the SINR of the HS-DSCH in dB, and γ0 is the HS-DSCH SINR to which CQI=0 corresponds. If it is assumed that the CQI feedback indicates that received SINR for the HS-DSCH, when the base station allocates (Ec/Ior)HSDSCH of power to transmit HS-DSCH, is γHSDSCH, and the target received SINR for E-HICH is γEHICH, then, the transmit power allocation needed to satisfy the target received SINR for E-HICH is:
            (                        E          c                /                  I          or                    )        EHICH    =                    (                              E            c                    /                      I            or                          )            HSDSCH        ⁢                  (                              γ            EHICH                                γ            HSDSCH                          )            .      Converting the above equation to dB,(Ec/Ior)EHICH,dB=(Ec/Ior)HSDSCH,dB+(γEHICH)dB−(γHSDSCH)dB=(Ec/Ior)HSDSCH,dB+(γEHICH)dB−CQI−γ0 The equation above can be used to compute the required power allocation factor of E-HICH. The transmit power for E-HICH is then:(PEHICH)dB=(PBS)dB+(Ec/Ior)EHICH,dB where (PBS)dB is the total base station power in dB. This works well when the cell that needs to send the E-HICH happens to be the serving cell for the HS-DSCH because in this case, CQI is readily available to the base station. The transmitted power of the E-AGCH and the E-RGCH can be determined in a similar fashion.
There are several problems with these known approaches, however. First, in practice, the downlink SHO gain of the DPCH is not known to the RNC. Thus, it is very difficult for the RNC to obtain a good estimate of z. As a result, performance of the E-HICH, the E-AGCH, and the E-RGCH is often not adequate in soft handoff mode, particularly for channels from non-scheduling cells. Second, for the CQI-based approach, the issue of determining the transmit power of the E-HICH, the E-AGCH, and the E-RGCH from the non-HS-DSCH serving cells is not addressed.
What is needed in the art is a solution for determining the transmit power of the E-HICH, the E-AGCH, and the E-RGCH that overcomes the shortcomings of the prior art. The present invention provides such a solution.